Today's large-scale multiprocessor computer systems include hundreds of CPUs and data storage devices having terabytes of memory. Data transmission between processors requires wide bandwidths and a data transmission speed of few Tb/s may be required by the end of this decade.
Presently electrical links are being used for establishing connections between chips. Electrical connections are reliable and it is possible to manufacture them in a cost efficient manner. However, electrical connects have fundamental physical limitations for high speed data transmission, which relate to electrical power requirements, transmission latency, achievable package density, etc.
Alternatively, data transmission between chips may be established using optical links, which have a number of advantages compared with electrical connections. Optical links may be used for high-speed data buses in computers and between signal processors such as those in mobile phones. The use of such “Optical Interconnects” (OI's) has also the added advantage that data transmission is not affected by electromagnetic interference.
Many OI structures have been proposed for high-speed data communication, including polymer waveguides, fibre image guides, fibre ribbons and free space optical connections using lenses and mirrors.
The use of high-speed optical fibre links for establishing chip-to-chip or “inter-chip” connections has the significant advantage that input and output platform size can be reduced to ˜ 1/500 compared with that required for electric connections, the throughput density can be increased by a factor of ˜1000, the power consumption can be reduced to ˜¼, and a data transmission speed exceeding 1 Tb/s is possible.
An OI incorporates an optical transmitter, an optical waveguide and an optical receiver. Key issues for designing OI's having a low Bit-Error Rate (BER) are transmission distance, properties of the optical waveguide such as dispersion, coupling losses, and properties of the optical source and receiver. However, for short optical links between chips losses of the optical waveguide and dispersion of guided light are negligible and consequently the remaining key-issues for the design of an OI are the properties of the source, the properties of the receiver and the coupling between them.
The optical source typically is provided in the form of a low-cost semiconductor laser, such as an edge-emitting laser or a vertical-cavity surface-emitting laser (VCSEL). A VCSEL typically is relatively compact and easy to integrate in one-dimensional or two-dimensional spaces, which is advantageous for small-scale designs that are required for high density parallel interconnects.
A laser beam emitted from a VCSEL typically has a circular intensity profile and initially a diameter of only a few μm. Even though the angle of divergence is relatively small (typically 10-15 degrees), a core of an optical fibre, into which emitted laser light may be coupled, needs to be positioned very close to the VCSEL in order to reduce coupling losses. Optical fibres with a core diameter of 62.5 μm (multimode) or 7-9 μm (single-mode) typically are used, and an end-face of the optical fibre needs to be positioned within ˜100 μm of the VCSEL along the axis direction and within a few microns in in-plane directions to minimize coupling loss. Such precise positioning of the optical fibre is cumbersome and expensive.
In order to enable high speed data transmission, such as data transmission at a rate of 10 Gb/s or more, optical fibres with smaller core diameters need to be used. Such smaller core diameters reduce the tolerances in positioning the optical fibre even further, which results in a significant technological challenge.